Oil wells (also referred to herein as “wellbores” or “boreholes”) are drilled to recover hydrocarbons (oil and gas) from subsurface formations. A drill string containing a drilling tubular carrying a drilling assembly (also referred to as the “bottomhole assembly” or “BHA”) is used for drilling the wellbore. BHA typically includes a large number of sensors that measure a variety of parameters relating to the formation or the drilling environment. Such sensors typically include resistivity sensors, gamma ray sensors, nuclear sensors, acoustic sensors, nuclear magnetic resonance sensors, formation testing sensors, sensors for estimating inclination, azimuth, etc. The BHA also includes one or more processors to process the downhole measurements and transmit in real-time selected information relating to the formation and other parameters to the surface via a telemetry unit. The telemetry method most often used is mud pulse telemetry, wherein signals are transmitted through the drilling fluid (or mud) circulating through the drill string. Such telemetry methods are capable of transmitting only a few pulses per minute. Therefore, a large amount of downhole data is often stored in one or more data storage devices (such as a solid state memory device) and only certain results obtained from such data are transmitted in real-time to the surface. The data stored in the downhole memory device (“memory data”) is retrieved when the drill string is tripped out of the wellbore and processed to estimate the various properties of interest and to generate images of one or more parameters. The quality level of downhole data or the images obtained from such data is evaluated after drilling is completed. The quality assessment of such data or images is usually subjective. It is therefore desirable to provide apparatus and methods for objectively assessing the quality level of real-time data and data retrieved from the memory.